spin contamination

[lyf-ustc]

Unrestricted quantum chemical calculations lead to spin symmetry breaking, resulting in spin contamination. The restricted open- shell Kohn- Sham (ROKS) approach is an alternative to describe the open- shell singlet excited state and to predict charge transfer and core excitations. It performs the self- consistent field (SCF) procedure simultaneously on both the “mixed” singlet and triplet states and removes the spin contamination of the open- shell singlet (OSS) state, which is impossible in broken- symmetry (BS)–density functional theory (DFT), marking its principal deficiency.

In this articlehttps://www.science.org/doi/10.1126/sciadv.adq7954, they use crystal software with PBE0 and HSE06 calculations for finding more Precise magnetic coupling exchange. But I test case (2D-PLY-trigulene), I found ABACUS will convergence to the wrong magnetic ground state. PBE0/DZP basis.

INPUT

INPUT_PARAMETERS calculation scf basis_type lcao pseudo_dir ./ orbital_dir ./ nspin 2 scf_nmax 100 ntype 2 vdw_method d3_bj dft_functional pbe0 scf_thr 1e-6 out_mul 1 symmetry 0

KPT

K_POINTS 0 Gamma 7 7 1 0 0 0

STRU

ATOMIC_SPECIES C 1.000 C_ONCV_PBE-1.0.upf H 1.000 H_ONCV_PBE-1.0.upf

NUMERICAL_ORBITAL C_gga_9au_100Ry_2s2p1d.orb H_gga_9au_100Ry_2s1p.orb

LATTICE_CONSTANT 1.8897259886 # 1.8897259886 Bohr = 1.0 Angstrom

LATTICE_VECTORS 12.4532003403000004 0.0000000000000000 0.0000000000000000 -6.2266001701000002 10.7847878530999992 0.0000000000000000 0.0000000000000000 0.0000000000000000 20.0000000000000000

ATOMIC_POSITIONS Direct C // Element type 0.0 26 0.534519953 0.465479993 0.5 mag 1 0.534520025 0.069039973 0.5 mag 1 0.930960062 0.465480038 0.5 mag 1 0.465480018 0.534519988 0.5 mag -1 0.465480038 0.930960075 0.5 mag -1 0.069039973 0.534520033 0.5 mag -1 0.666666675 0.333333349 0.5 mag 0 0.333333349 0.666666699 0.5 mag 0 0.664690039 0.136130004 0.5 mag -1 0.863869996 0.528560048 0.5 mag -1 0.471439971 0.335309996 0.5 mag -1 0.863870007 0.335309996 0.5 mag -1 0.471439965 0.136130004 0.5 mag -1 0.664690052 0.528560048 0.5 mag -1 0.335310024 0.863870044 0.5 mag 1 0.136129979 0.471439978 0.5 mag 1 0.528560023 0.664690029 0.5 mag 1 0.136129968 0.664690029 0.5 mag 1 0.528560059 0.863870044 0.5 mag 1 0.335309999 0.471439978 0.5 mag 1 0.534769992 0.267390027 0.5 mag 1 0.732609974 0.267380013 0.5 mag 1 0.73262002 0.465230008 0.5 mag 1 0.465230021 0.732609999 0.5 mag -1 0.267390031 0.732620035 0.5 mag -1 0.267380012 0.534770018 0.5 mag -1

H 0.0 12 0.08808 0.370519996 0.5 mag 0 0.62948 0.717559997 0.5 mag 0 0.282440012 0.911919993 0.5 mag 0 0.62948002 0.911919993 0.5 mag 0 0.282440014 0.370519996 0.5 mag 0 0.088079978 0.717559997 0.5 mag 0 0.911919982 0.629480008 0.5 mag 0 0.370519983 0.282440007 0.5 mag 0 0.717560018 0.088079989 0.5 mag 0 0.370519971 0.088079989 0.5 mag 0 0.717560026 0.629480008 0.5 mag 0 0.911919967 0.282440007 0.5 mag 0

so i hope developer can solve this probelm. ABACUS with hybrid functional make a very big spin contamination for this magnetic polymers.

[mohanchen]

Thanks for the report! We will see what we can do about it.

[mohanchen]

It seems that the website you provided did not work, could you please double check?

[lyf-ustc]

Density functional theory calculations in article (Prediction of metal- free Stoner and Mott- Hubbard magnetism in triangulene- based two- dimensional polymers)

[kirk0830]

it is not appropriate to talk about spin contamination in DFT. But do you mean your DFT calculation converges to an unexpected state?